1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a diode used to perform fine adjustment on the value of resistance in an electronic circuit by zapping.
2. Description of the Related Art
There are often cases where fine adjustment is performed on the value of resistance involved in an integrated circuit by an overvoltage applied to a P-N junction diode provided in the circuit to cause breakdown in a P-N junction thereof. This fine adjustment is called "zapping" or "zener zap", and a diode for use in such zapping is called a "zapping diode".
There is illustrated in FIG. 1 a top plan view showing a structure of a conventional zapping diode, while FIG. 2 is a cross-sectional view taken on line II--II in FIG. 1. As shown in these figures, a P-type region 12 is formed to be encompassed by a device-isolation layer 10 in an N-type semiconductor substrate 11. Formed in the surface of P-type region 12 in a P+-type region 13 and an N+-type region 14, both having a high impurity concentration. Aluminum interconnections 15 are provided respectively in electrical connection to the P+-type region 13 and the N+-type region 14 through each contact hole 17.
FIG. 3 is a circuit diagram showing an example of a circuit including a zapping diode. A resistance 31 is connected in parallel with the zapping diode 32. When zapping is performed to finely adjust the value of resistance 31 which is involved in an electronic circuit, an overvoltage under reverse-bias 20 is applied to the zapping diode 32, as shown in FIG.2. By thus zapping, an electronic circuit having a breakdown path or an electric current path which has an appropriate value of residual resistance is formed in the zapping diode.
However, a problem remains in the above-stated structure. When the P-N junction is broken down by a reverse-biased voltage, there exists no directionality in the junction breakdown, as shown in a top plan view of FIG. 4 wherein the arrows 41, 42, and 43 indicate directions of junction breakdown, for example The location where the electric current passage forms varies. That is, sometimes it forms along the solid line 41 and othertimes along the broken line 42 or 43. This is because a breakdown path, or an electric current path, it is to be formed where the electric field is mostly concentrated, with the result that breakdown occurs in direction that inconsistently differs from time to time. As a result, there may be a case where the length of the current path, which forms as a result of the junction breakdown from the P+-type region 13 to the N+-type region 14, is variable, and accordingly a consistent desired value of residual resistance is not obtainable by usual zapping. Moreover, the voltage at which junction breakdown occurs differs from time to time, so that it may cause damage to the other circuit elements which are connected to the zapping diode.
In this manner, the conventional zapping diode has the problem that the value of residual resistance after breakdown inconsistently varies due to the absence of directionality of breakdown in a P-N junction.